How NES Graphics work
The Basics
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All the graphical information is stored within the 16kb memory of
the Picture Processing Unit (PPU).
The first area of PPU memory is known as the "Pattern Tables." The
pattern tables are 8kb in size, which is usually split in half, one part
for the background, the other for sprites. The usage of the pattern tables
is determined by the PPU control registers. All graphics are stored in 8x8
pixel "tiles" within the pattern tables which are arranged to form
backgrounds and sprites. The pattern tables (also known as VROM) contains
half the graphics information for these tiles. The NES is capable of
displaying 16 colors at once, since each pixel is 4 bits that are a lookup of
the palette. The pattern table contains the low two bits, and the upper two
either come from the attribute table (for the background) or from sprite
ram (for sprites).
Some roms contain what is known as CHR-RAM. CHR-RAM stands for
CHaRacter RAM and contains 8kb banks of pattern tables which can be swapped
in and out of the PPU via an MMC. This saves the trouble of copying it
from within the program code to the PPU like games without CHR-RAM do.
The second area of PPU memory is known as "Name Tables." There are
usually two, although there are addresses for 4. The other two are mirrors
of the actual tables, which is determined by the mirroring bit of the .NES
header. Each name table is 960 bytes, which corresponds with 960 8x8 tiles
that make up the background. The background is 32x30 tiles, or 256x240 pixels.
In a NTSC NES, however, the top and bottom 8 lines are not displayed, making
the actual resolution 256x224. Note that some games use only one name table,
and others can use 4. These settings are dependant on the memory mapper used.
Paired with each name table is an attribute table. These tables
contain the upper two bits of each pixel's color that are matched with the
lower two bits from the pattern table. However, the attribute table is only
64 bytes in size, meaning that each byte contains the upper two bits for a
group of 4x4 tiles (or 32x32 pixels). This puts a limitation on your choice
of colors in the background. However, you can get around the attribute table
limitation by using MMC5, which allows you to use 4 name tables and the
upper two bits for each individual tile. For more information, read
Y0SHi's very informative NESTECH.DOC
Also stored in PPU memory are two 16 color palettes. One is used for
the background, the other for sprites. These are not actual rgb palettes, but
lookup tables of a 256 color palette that the PPU translates into tv signals.
You can modify the values stored in the palette, allowing you to create many
effects such as fades, flashing, or transparency.
There is an independent area of memory known as sprite ram, which is
256 bytes in size. This memory stores 4 bytes of information for 64 sprites.
These 4 bytes contain the x and y location of the sprite on the screen, the
upper two color bits, the tile index number (pattern table tile of the sprite),
and information about flipping (horizontal and vertical), and priority
(behind/on top of background). Sprite ram can be accessed byte-by-byte through
the NES registers, or also can be loaded via DMA transfer through another
register.
There are various other aspects of the PPU that can be controlled via
several nes registers. Here's a list of registers and what they do:
PPU Control register 1 (PPUCTRL0):
* Selects the Name table to display
* Sets the ppu address increment (for reading/writing)
* Sets the address within the pattern table for sprite tiles
* Sets the address within the pattern table for background tiles
* Selects the sprite size (8x8 or 8x16)
* Sets whether to execute an interupt when drawing sprite 0
* Sets whether to execute an interupt during the Vblank period
PPU Control register 2 (PPUCTRL1):
* Sets the disply to color or mono-tone
* Sets whether to clip the left 8 pixels of the background
* Sets whether to clip sprites within the left 8 pixels of the background
* Sets whether to display the screen or not
* Selects the screen background color (black, red, blue, green)
PPU Status register (PPUSTAT):
* Returns whether PPU is in a Vblank period
* Returns whether there are more than 8 sprites on the current scanline
* Returns whether sprite 0 has been drawn on the current scanline
Background Scroll (BGSCROLL):
* Sets the horizontal and vertical scroll (written to twice)
